Catalysts produced by depositing a precious metal component, such as Pt, Pd, and Rh, on an activated alumina are generally used for purifying exhaust gases from internal combustion engines. The catalyst can purify hydrocarbons (HC), carbon monoxide (CO), and NOx simultaneously, and is called as “Three Way Catalyst or TWC”.
The TWC is operated effectively under the conditions nearby the theoretical air-fuel ratio (A/F), but has a problem that removal of NOx is insufficient in oxygen rich conditions for example lean condition such as exhaust gases from diesel engines.
Further, the exhaust gas of a diesel engine includes particle matters formed of carbon, a soluble organic fraction (SOF), and sulfates in a large amount, which particles harm human bodies. It, therefore, constitutes one object of the legal control. When the ordinary TWC is used for treating the exhaust gas of a diesel engine, the catalyst used is required to reduce the particle matters therein.
It is well known a copper zeolite catalyst (Cu-ZSM5), in which copper is deposited on a porous zeolite, as the catalyst for purifying NOx under lean conditions (JP-A-60-125250). When the catalyst is used for treating exhaust gases in particular from diesel engines, there are problems that particle matters cannot be reduced, since it causes SO2 to oxidize and form sulfates under high temperatures of above 450° C.